This invention relates to olefin isomerization. In one of its more specific aspects, this invention relates to selective isomerization of n-pentenes.
More particularly, the present invention relates to a process for the preparation of useful hydrocarbons by catalytic conversion of n-pentenes.
TAME (tertiary amyl methyl ether) is an effective octane booster as well as a source of oxygenates in gasoline that are effective in reducing CO and hydrocarbon emissions. It is made from 2-methylbutenes and methanol. The present sources of 2-methylbutene for TAME production are mainly from by-products of steam cracker, catalytic cracker and cokers. However, these supplies are limited. Other possible sources are by isomerization of n-pentenes taken from steam or catalytic crackers and by dehydrogenation of isopentane produced by isomerization of n-pentane.
Olefin isomerization processes can be directed towards either skeletal isomerization or double bond isomerization. Skeletal isomerization is concerned with reorientation of the molecular structure in respect to the formation or elimination of side chains. Double bond isomerization is concerned with relocation of the double bond between carbon atoms while maintaining the backbone of the carbon structure. Most isomerization processes give rise only to double bond isomerization.
It is frequently necessary to convert olefins into other olefins having a different skeletal arrangement. For example, normal butenes are converted into isobutene for polymerization, alkylation, disproportionation or for the production of MTBE. Similarly, normal pentenes must be converted to isoamylenes prior to dehydrogenation to isoprene.
While a number of catalytic materials possess some activity for such a conversion, not all possess sufficient selectivity to be economical. Because the feeds are generally the relatively reactive olefins, many catalysts cause undesirable side reactions such as oligomerization polymerization or cracking. Consequently, there is a continuing interest in the development of new or improved skeletal isomerization catalysts and processes for isomerizing alkenes to improve efficiencies and to give optimum results for various industrial requirements. A comprehensive review is provided by V. R. Choudhary in "Catalytic Isomerization of n-Butene to Isobutene," Chem. Ind. Dev, pp. 32-41 (1974).
It is generally known that n-paraffins with, for example, 4 to 7 carbon atoms can be converted to the corresponding isomeric paraffins by using suitable bifunctional (acid and metal) catalysts in the temperature range of from 100.degree. to 250.degree. C. Examples of this process are the numerous isomerization processes used in the petrochemical and mineral oil industries for increasing the octane number of light, paraffinic mineral oil fractions. Furthermore, it is known that, in contrast to this, olefins of the same number of carbon atoms cannot be converted to the corresponding isoolefins except under difficult conditions, for example at very high temperatures and with poor yield. The attempts hitherto described in the literature for the direct isomerization of the skeleton of e.g. n-butene to give isobutene or e.g. of n-pentene to give isoamylenes over catalysts arranged in a fixed bed are characterized by initially relatively low yields and selectivities, which can diminish and deteriorate further after a short period of operation, often after only a few hours. The deterioration in the yields and selectivities is generally attributed to the loss of actively effective catalyst surface or to the loss of active centers. In addition to this, high coking rates, formation of oligomers and cracking reactions are observed.
As is known, pentenes exist in six isomers. Conversions between the cis and trans 2-pentenes are known as geometric isomerization, whereas those between 1-pentene and the 2-pentenes are known variously as position isomerization, double-bond migration, or hydrogen-shift isomerization. These three isomers are not branched and are known collectively as normal or n-pentenes. Conversion of the n-pentenes to the isoamylenes, i.e., 2-methyl-1-butene, 2-methyl-2-butene or 3 methyl-1-butene, which are branched isomers collectively known as isoamylenes, is widely known as skeletal isomerization.
Also, isoamylenes or more specifically, the 2-methylbutenes have become more and more important recently as one of the main raw materials used in the production of tertiary amyl methyl ether (TAME), an environmentally-approved octane booster and source of oxygen in gasoline. However, processes for the skeletal isomerization of olefins, e.g., to produce isoamylenes, are relatively non-selective, inefficient, and short-lived because of the unsaturated nature of these compounds. On the other hand, positional and skeletal isomerization of paraffins and alkyl aromatics are fairly well established processes, in general utilizing catalysts typically comprising metallic components and acidic components, under substantial hydrogen pressure. Since paraffins and aromatics are relatively stable compounds, these processes are quite successful. The heavier the compounds, in fact, the less severe the operating requirements. Olefins, however, are relatively unstable compounds. Under hydrogen pressure, they are readily saturated to the paraffinic state if a metal component is present in the catalyst.
Furthermore, in the presence of acidity, olefins can polymerize, crack and/or transfer hydrogen. Extensive polymerization would result in poor isoolefin yields, and short operating cycles. Similarly, cracking would reduce yield. Hydrogen transfer would result in saturated and highly unsaturated compounds, the latter being the common precursors for gum and coke. Any theoretical one step process for producing skeletal isomers of, for example, n-pentenes, would have to be concerned with the unwanted production of olefin oligomers, saturates and cracked products.
Skeletal isomerization of olefins is known to be accomplished by contacting unbranched or lightly branched olefins with acidic catalysts at elevated temperatures. The process is generally applicable to the isomerization of olefins having from about 4 to about 12 carbon atoms, and is especially applicable to olefines having from about 4 to about 6 per molecule. The process may be used to form isobutene from normal butenes, methylpentenes and dimethylbutenes from normal hexenes, and so forth.
Thus, among the objects of this invention are improved catalysts for the skeletal isomerization of n-pentenes to form isoamylenes on pretreated zeolite catalysts.
Other objects and advantages of the invention will be apparent from the following description, including the drawing and the appended claims.